In conventional flat panel display technology, a low resolution image can be displayed upon a higher resolution screen by limiting the display to a portion of the screen having the same resolution as the image, or by expanding the image by replication of pixels horizontally or vertically or both. Typically, vertical lines may be added by periodically replicating the pixels of the preceding line to provide the desired expansion factor.
However, horizontal expansion of character data is not provided because the character clock is typically used to clock the display, this being a submultiple of the pixel clock rate. Thus, the aspect ratio of text screens may be distorted by the vertical expansion without a corresponding horizontal expansion.
Image expansion is a traditional problem in video image processing. Many systems exist for applications such as image display and printing, but typically the methods employed are complicated and cannot be performed in real time, usually requiring large amounts of memory, and therefore cannot be considered suitable for a low cost panel display video subsystem.
Without expansion, for example, a VGA 640.times.480 pixel screen output would be displayed in a reduced area on a 1024.times.768 SVGA flat panel display, which would defeat the purpose of the higher resolution screen. This type of display method would leave 384 pixels blank at the right of the screen and 288 blank lines at the bottom of the page. To increase the usable screen area, both horizontal and vertical expansion, preferably by the correct scale factor, are required. To expand a 640.times.350 EGA display to a VGA display would require vertical expansion from 350 lines to 480 lines, which could be achieved by replication of 130 of the 350 original lines.
Many controller chips support such vertical expansion methods. However, none support a similar horizontal expansion method, as they are typically clocked at 1/8 of the pixel rate in VGA architecture. Eight pixels at a time are transmitted to the screen to provide the elements of one row of specific text character in text modes, or graphic information in graphics display modes. This results in a distorted aspect ratio such as with 640.times.480 images expanded to 640.times.768 pixels by replication of 288 lines.
Another approach in flat panel technology is to replicate pixels vertically using the panel logic to simultaneously activate two row drivers at selected times. The column drivers are usually split into several chips and all of them must be driven simultaneously during one line scan, making it impossible to replicate pixels horizontally.
What is needed is an efficient system to permit horizontal expansion of an image on a flat panel display by a variable scaling factor.